go/gcimporter15: update to match std lib gcimporter (fix build)

TBR=adonovan

Change-Id: Ib2464def48932e0d0fc24f67c76a10e8918acb9d
Reviewed-on: https://go-review.googlesource.com/27235
Reviewed-by: Robert Griesemer <gri@golang.org>
2 files changed